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authorMarcelo Tosatti <mtosatti@redhat.com>2013-07-10 22:21:57 -0300
committerPaolo Bonzini <pbonzini@redhat.com>2013-07-18 12:29:30 +0200
commite04c5d76b0cfb66cadd900cf147526f2271884b8 (patch)
treea303b66c1fc1ae7786148d2f50e8b0b86290162b /arch/x86/vdso
parentb3897a49e22fc173efa77527a447c714f753f681 (diff)
downloadop-kernel-dev-e04c5d76b0cfb66cadd900cf147526f2271884b8.zip
op-kernel-dev-e04c5d76b0cfb66cadd900cf147526f2271884b8.tar.gz
remove sched notifier for cross-cpu migrations
Linux as a guest on KVM hypervisor, the only user of the pvclock vsyscall interface, does not require notification on task migration because: 1. cpu ID number maps 1:1 to per-CPU pvclock time info. 2. per-CPU pvclock time info is updated if the underlying CPU changes. 3. that version is increased whenever underlying CPU changes. Which is sufficient to guarantee nanoseconds counter is calculated properly. Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com> Acked-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Gleb Natapov <gleb@redhat.com>
Diffstat (limited to 'arch/x86/vdso')
-rw-r--r--arch/x86/vdso/vclock_gettime.c16
1 files changed, 8 insertions, 8 deletions
diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index c74436e..72074d5 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -85,15 +85,18 @@ static notrace cycle_t vread_pvclock(int *mode)
cycle_t ret;
u64 last;
u32 version;
- u32 migrate_count;
u8 flags;
unsigned cpu, cpu1;
/*
- * When looping to get a consistent (time-info, tsc) pair, we
- * also need to deal with the possibility we can switch vcpus,
- * so make sure we always re-fetch time-info for the current vcpu.
+ * Note: hypervisor must guarantee that:
+ * 1. cpu ID number maps 1:1 to per-CPU pvclock time info.
+ * 2. that per-CPU pvclock time info is updated if the
+ * underlying CPU changes.
+ * 3. that version is increased whenever underlying CPU
+ * changes.
+ *
*/
do {
cpu = __getcpu() & VGETCPU_CPU_MASK;
@@ -104,8 +107,6 @@ static notrace cycle_t vread_pvclock(int *mode)
pvti = get_pvti(cpu);
- migrate_count = pvti->migrate_count;
-
version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
/*
@@ -117,8 +118,7 @@ static notrace cycle_t vread_pvclock(int *mode)
cpu1 = __getcpu() & VGETCPU_CPU_MASK;
} while (unlikely(cpu != cpu1 ||
(pvti->pvti.version & 1) ||
- pvti->pvti.version != version ||
- pvti->migrate_count != migrate_count));
+ pvti->pvti.version != version));
if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
*mode = VCLOCK_NONE;
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